Generally, ventilation facilities consist of large, centrally positioned and centrally controlled, assemblies with large energy demands, and with an extensive system of ducts for ingoing and outgoing air. The duct system branches off towards the locations of consumption, where preadjusted air terminal devices control the amount of air that a room is expected to require. The air ducts often cross fire cells, and in these cases they are generally equipped with automatic fire valves. Further, the ducts need to be sound dampened, partly because of overhearing between rooms through the air terminal devices and the duct system, and partly due to the sounds that are generated by fans and to the sounds arising in the usually strictly controlled air terminal devices. The facilities are equipped with filters against dust and small particles, but they still get contaminated and have to be cleaned. Presently, there is an increasing concern about how the indoor environment is affected by air passing over long distances in more or less clean ducts, and the kind of spreading of odours and bacteria it may cause.
Large fan assemblies in day care centres, schools, and the like, demand large spaces in the form of separate fan rooms, which have to be sufficiently large and accessible to function as worksite for the operating staff running the facility. Further, buildings are usually constructed with larger ceiling height than required by the user, in order to generate space for the installation of the system. If this space is not concealed with an extra inner ceiling, visible ducts arise, which then have to be made aesthetically appealing. Further, dust-collecting surfaces, which are difficult to clean, arise through such installations.
In order to control a ventilation facility of this magnitude, computer controlled regulating equipment, so called CSC:s (=Computerized Sub-Centrals) are installed, which, apart from the controlling computer itself, consists of measuring equipment/transmitters positioned throughout the building. These are connected to the CSC through separate lines, and form a separate network of cables in the building. In addition to the control unit, the ventilation assemblies demand electrical power, with separate feeding of power to the fan room, where the ventilation facility often also is equipped with a separate distribution box. If insulated ventilation ducts are placed in a sheltered outside position, for example, on an unheated attic, they will not use any indoor space, but by being connected between indoor rooms and the ventilation unit in which heat exchange and heating tales place, the walls of the ducts should be viewed as an extension of the climate shell of the building. The costs for the ventilation contract usually constitute about 15%, the contract for the controlling units approximately 15%, while the extra building area for the equipment room constitutes about 3–5%. Finally the extra ceiling height in order to make room for the installation may be added, which entails another 3–5%. In addition to this are the costs of the electrical power facility for the ventilation device.
Such ventilation devices may also be equipped with heat-exchangers, see U.S. Pat. No. 5,024,263, which shows a typical example of a large, centrally placed and centrally controlled device, intended to be connected to a ventilation system, in which a large amount of energy is used simply to transport the air. Being able to balance and control the airflows are of course desirable also in large systems. However, the regulation of airflows in said document is limited in direct connection to the unit, and within the unit. It does not guarantee a balanced flow of air into each room. An unscheduled adjustment, e.g., of an valve of ingoing air, or by the opening of a window when the wind blows head on, in one of the rooms that is supplied, together with several other rooms, from the same duct of ingoing air, would affect the pressure conditions in the duct so that, due to the pressure increase, more ingoing air would flow to the other rooms along the duct of ingoing air. If the decreased flow also affects the flow sensor in the centrally placed ventilation device into changing the fan output, or some valve flap, in order to compensate, the air balance would be further changed for all rooms supplied by the same duct for ingoing air.
A further drawback of this ventilation system according to prior art is the bypass-arrangement, which allows outgoing air to flow across to the ingoing side, which means pollution of the ingoing air. Contagion could be transferred between rooms, and cooking fumes and air from toilet facilities could be spread throughout the entire building. Internal leakage in heat-exchangers is another problem in large systems with an extensive flap system, which may allow leakage between ingoing and outgoing air.
Through SE-B-470,194, is prior known a method for controlling the amounts of air across a heat-exchanger, which method is claimed to achieve a kind of balance point in the airflows, oscillating within the area of maximum efficiency. The method assumes that a temperature difference arises when a medium has passed the heat-exchanger, which, of course, is normal during heat exchange. The control of the fan output is based on temperature differences. Consequently, there are no readable values which can regulate the output of the fan when the outdoor and indoor temperatures are the same or similar, i.e. in the interval where no heat exchange takes place.
DE 2,906,837 A1 discloses a heat-exchanger according to the counter-flow principle, intended for ventilation of rooms, and where the heat-exchanger is arranged in a duct below and along a window. The heat-exchanger takes in fresh air from the outside, and heats it by the outgoing air. The difficulty, so far, has been to scale down the dimensions sufficiently to house the heat-exchanger and the control equipment within the short duct defined by the thickness of the wall.